X-RAYS FROM THE SUN — FRIEDMAN 255 



Observations over more than two sunspot cycles have clearly estab- 

 lished correlations between the fluxes of ionizing radiation and active 

 centers on the sun. If the ionizing radiation were uniformly dis- 

 tributed over the face of the sun, an eclipse would lead to a smooth 

 decline in the ionospheric electron density to a minimum value at 

 totality, followed by a smooth recovery to normal in very much the 

 same fashion, followed by the visible light curve. Instead, an irregu- 

 lar course of ionospheric electron-density changes has been noted in 

 almost all observations conducted during eclipses. Monthly averages 

 of critical frequencies show detailed agreement with the pattern fol- 

 lowed by monthly values of sunspot numbers, indicating that at 

 least part of the ionizing flux emanates from the vicinity of sunspots. 

 Prior to 1958, however, no direct identification of localized sources of 

 X-ray emission in the corona had been made. 



The eclipse of October 12, 1958, offered an opportunity to launch 

 rockets bearing ultraviolet and X-ray detectors to observe the dis- 

 tribution of emission sources over the disk and to determine whether 

 any residual emission of X-rays or ultraviolet radiation was detectable 

 at totality. During the totality phase of an eclipse, the E-region of 

 the ionosphere does not disappear completely as would be expected 

 if the source of the ionizing radiation were totally obscured and 

 recombination were very fast. The residual ionization could be at- 

 tributed to a sluggishness of the recombination process or to a portion 

 of the ionizing radiation originating at sufficient height in the corona 

 to bypass the edge of the moon. 



The rocket experiment was carried out from shipboard near the 

 Danger Islands of the South Pacific. Solid-propellant rockets were 

 mounted on the helicopter deck of the U.S.S. Point De-fiance and were 

 launched at the appropriate times and in such a direction as to carry 

 them through the eclipse shadow at E-region altitudes. Each rocket 

 was equipped with X-ray detectors sensitive to two wavelength bands, 

 8 to 18 A. and 44 to 60 A., and a Lyman-a ionization chamber. Signals 

 from these detectors and from aspect indicators were telemetered to 

 the ground station aboard ship throughout the flight. Two rockets 

 were launched during totality and indicated about 0,05 percent residual 

 Lyman-a flux and from 10 to 13 percent residual X-ray flux. 



A second objective of the experiment was to identify localized 

 sources of emission over the disk. Figure 2 shows the optical dis- 

 tribution of active regions on the day of the rocket eclipse experiment. 

 The area of the disk near the east limb contained a number of active 

 regions identified by plages, whereas an equivalent area bordering 

 the west limb was almost free of activity. Rockets were fired so as to 

 observe exposed crescents on the east and west limbs before second 

 contact and after third contact, as marked by the curves NN8.59F 



